1. Field of the Invention
The invention relates to a receiver having an RF input section for receiving for applying an RF input carrier-modulated modulation signal. The RF input section is coupled to a multiplier circuit and to a phase-locked loop (PLL) with a signal path incorporating a phase detector, a loop filter, a first dc decoupling circuit and a controlled oscillator having an in-phase and a quadrature output via which local in-phase and quadrature carriers are applied to the multiplier circuit and the phase detector, respectively, and a signal generator for generating a local auxiliary pilot and a pilot detector for detecting the local auxiliary pilot, an output of which is coupled to the controlled oscillator via a low-pass filter.
2. Description of the Related Art
A receiver of this type is known per se, for example from European Patent Specification no. 184873 (corresponding to U.S. Pat. No. 4,631,499.
The known receiver is a directly mixing AM PLL synchronous receiver whose multiplier circuit is used for synchronous detection of a modulation signal which is amplitude-modulated on the RF input carrier. The local mixing carrier required for the synchronous detection is obtained by means of the controlled oscillator incorporated in the phase-locked loop and should be phase-synchronous with the RF input carrier for a correct synchronous detection. The local mixing carrier is therefore also referred to as local in-phase carrier.
As is known, the loop signal, i.e. the signal in the phase-locked loop, undergoes an unwanted dc offset which is substantially caused by circuit and/or bias voltage asymmetries and cannot be completely avoided in practice. To prevent this unwanted dc offset from disturbing a correct phase control of the controlled oscillator, particularly at comparatively small amplitudes of the RF input carrier, said RF input carrier and the local quadrature carrier of the controlled oscillator modulated in amplitude by means of the auxiliary pilot are applied to the phase detector of the known directly mixing AM synchronous receiver. Consequently, the phase detector supplies a desired mixing product at a frequency which is equal to the auxiliary pilot frequency and which varies in amplitude with the phase difference deviating from 90.degree. between the RF input carrier and the local quadrature carrier. DC components are removed from this desired mixing product in said first dc decoupling circuit and subsequently applied to the pilot detector for synchronous amplitude detection. A dc signal whose amplitude varies with the last-mentioned phase difference is obtained at the output of the pilot detector. This dc phase difference signal is applied as an oscillator control signal to the controlled oscillator. Due to the dc offset suppression in the first dc decoupling circuit, this oscillator control circuit and hence the phase lock between the local carriers and the RF input carrier is comparatively insensitive to said unwanted dc offsets.
However, due to oscillator radiation and other effects of parasitic crosstalk, the unmodulated oscillator signal occurs with a crosstalk-dependent amplitude at the RF input of the receiver. This parasitic oscillator signal is multiplied in the phase detector by the oscillator signal which is amplitude-modulated by means of the auxiliary pilot, which results in an unwanted mixing product at a frequency which is the same as that of the auxiliary pilot. This unwanted mixing product is inseparably superimposed on said desired mixing product and disturbs the phase difference information in the desired mixing product. Both mixing products jointly pass the first dc decoupling circuit, with the unwanted mixing product at the output of the pilot detector resulting in a parasitic dc offset disturbing the desired dc phase difference signal. Since this parasitic dc offset increases with the frequency of the oscillator signal, a phase error, increasing with the frequency, between the local carrier and the RF input carrier occurs in the known directly AM synchronous receiver.
The dc offset reduction mode used in the known AM PLL synchronous receiver is therefore unsuitable for use in receivers for the reception of signals which are modulated on a much higher carrier frequency than that of the conventional RF AM radio signals which are of the order of at most 1 MHz. Examples are FM PLL synchronous receivers for receiving the now conventional FM radio signals at an RF carrier of the order of 100 MHz, and FM receivers including a PLL FM demodulator tuned to a comparatively high intermediate frequency such as, for example TV satellite receivers.
Moreover, the 3 dB loop passbandwidth of the phase-locked loop of the known AM PLL synchronous receiver is considerably smaller than that of FM receivers and therefore presents fewer problems as regards noise behavior, stability and selectivity.